The present invention generally relates to a method of starting a thermal power plant. More particularly, the invention is concerned with a thermal power plant start-up method which can satisfy at least one of various basic conditions for starting up the thermal power plant while abiding by the constraint conditions imposed on the plant start-up.
According to a hitherto known method of starting a thermal power plant, a start-up schedule is prepared by incorporating the initial amount of fuel to be charged into a boiler, temperature-up and pressure-up of main steam as a function of time, as well as load-up and speed-up of turbines as a function of time in consideration of the rest time of he power plant preceding to the start-up and temperature states of instruments and machineries, the start-up schedule thus prepared being then executed by control systems provided in various systems of the thermal power plant. One of the most typical methods is described in an article entitled "Thermal Stresses Influence Starting, Loading of Bigger Boilers, Turbines" in "Electrical World", Vol. 165, No. 6.
The known method mentioned above resides in that the start-up schedule is definitely determined in dependence on the initial states of a limited number of locations of the power plant. More specifically, according to this known method, the warm-up time of the steam turbine and the load variation rate are determined in accordance with the initial values of boiler steam pressure, boiler exit steam temperature and the temperature of steam turbine casing by correspondingly determining the speed-up rate of the steam turbine and the initial load and holding the speed and load at respective constant values. Since dispersion of the temperature-up characteristics of the steam generated by the boiler is accommodated in a margin of the start-up schedule according to this method, the start-up schedule as prepared tends to be excessively lengthy. This, in turn, means that the starting loss (i.e. loss involved in the start-up process of the thermal power plant) also tends to be increased. Other methods known heretofore are disclosed in U.S. Pat. Nos. 3,446,224 and 4,228,359. These known methods are directed to quick starting of the steam turbine by monitoring on-line the thermal stress produced in the steam turbine on the real time basis. However, according to these methods, the starting loss can not necessarily be reduced to a minimum. Further, consideration is paid neither to the method of abiding by the preset or commanded starting time nor to the method of starting the boiler system.
As the hitherto known method, there can be mentioned a method disclosed in Japanese Patent Application Laid-Open No. 157402/1984 (JP-A-59-157402). This method is directed to the quick temperature-up of the steam generated by the boiler by monitoring on-line the thermal stress produced in the boiler on a real time basis. However, this method does not necessarily reduce the starting loss to a minimum. Besides, no teachings are found as to the method of abiding by the designated starting time and the start-up of the turbine.
There has been proposed a thermal power plant starting system in which a turbine bypass system is provided and in which the plant start-up is effectuated by starting first the intermediate pressure turbine (reference may be made to Japanese Patent Laid-Open Application JP-A-57-93611, Published on June 10, 1982). According to this method, initial steam admission is made to the intermediate pressure turbine under the condition that the predetermined metal matching condition is fulfilled, to thereby speed up the turbine at a predetermined rate. However, since the temperature-up characteristics of the boiler differ from one to another starting, the speed-up rate is determined with a large margin for accommodating the difference or deviations of the temperature-up characteristics. Consequently, the speed-up of the power plant by means of the intermediate pressure turbine takes excessive time. As a result, the initial steam admission to the high pressure turbine upon completion of the speed-up process tends to be accompanied with a delay relative to the temperature-up of the boiler, giving rise to thermal shock to the high pressure turbine. In the worst case, a chance for metal matching may be missed or the plant start-up will result in failure.
As will be seen, any one of the hitherto known methods is concerned with the quick starting system in which either only the boiler or alternatively only the steam turbine is considered. Combinations of these discrete methods can not always solve the basic problem in the thermal power plant, i.e. starting of the power plant within a predetermined or fixed time with the minimum starting loss or reduction of the starting loss and life shortening of the machinery or starting within the shortest time while satisfying various conditions as imposed when the whole thermal power plant is considered comprehensively. This is because an extremely strong mutual interference exists between the boiler and the steam turbine, which means that individual optimization of the boiler and/or steam turbine can not always lead to optimization of the whole system. The basic problems imposed on the start-up system for the thermal power plant are to realize the following requirements: